The present invention concerns the field of telecommunications and enables the connection of switching systems based on the ATM (Asynchronous Transfer Mode) technique to systems based on the STM (Synchronous Transfer Mode) technique. The progressive introduction of ATM type switches into the current network means that they must be able to work with STM type switches.
The STM technique is characterized by the multiplexing of several calls at 64 kbit/s onto the same PCM frame. The frame is subdivided into 32 time slots TS0 through TS31. Each TS is reserved for a specific call and has a data rate of 64 kbit/s. The start of the frame is identified by a known synchronization pattern included in TS0.
The ATM technique is characterized by the multiplexing onto a common medium of packets made up of, for example, 36 bytes identified by a header on 4 bytes and having a useful content of 32 bytes. A given header represents a given call. The data rate of the medium is independent of that of the calls that it carries.
Synchronous-asynchronous conversion of a PCM frame therefore involves:
In the Synchronous to Asynchronous direction:
storing 32 bytes of each channel to form the useful content of the packets, PA1 adding to each packet a header specific to each channel, PA1 multiplexing the various packets onto a common medium, PA1 identifying the channel by analyzing the header of the received packet, PA1 storing the useful content of the packets received, PA1 reading a byte in the memory and transmitting it on the PCM in each TS time.
In the Asynchronous to Synchronous direction:
The usual synchronous-asynchronous conversion systems convert a single digital channel, at low or high data rate, and include a packetizer and a depacketizer. The packetizer assembles a packet from the digital bit stream by counting incoming bits or bytes and then adds a label which identifies the service. The depacketizer carries out the converse operation and reconstitutes a recurrent bit stream from the information contained in the packets with the same label.